A ball screw is a mechanical element that converts rotational motion of a motor or the like into linear motion and is used for conveying, positioning, or the like of a semiconductor manufacturing device, an industrial robot, or a machine tool. Types of the ball screw include a tube type, an end-cap type, a deflector type, and the like.
In the tube type ball screw, a tube is bent into a U shape so as to serve as a circulation portion and is attached to a nut. After a ball that passed through the tube travels in one and a half round, two and half rounds, or three and half rounds of a screw groove, the ball again passes through the tube. Thus, one ball circulation circuit is constituted. In addition, in the end-cap type ball screw, circulation parts are attached to both ends of a nut. A through hole is provided in a nut body for allowing a ball to go back and forth. In the deflector type ball screw, a ball rolling between a screw shaft and a nut is circulated by a deflector (internal deflector) embedded in the nut, and a circulation circuit is formed in each lead.
FIGS. 14A and 14B illustrate a conventional deflector (an internal deflector type ball screw). This ball screw 1 includes a screw shaft 2 having a spiral screw groove 2a formed in an outer peripheral surface of the screw shaft 2, a nut 3 which is fitted over the screw shaft 2 and having a spiral screw groove 3a formed in an inner peripheral surface of the nut 3, a large number of balls 4 which are accommodated in a rolling path formed of the screw grooves 2a, 3a facing each other, and an internal-deflector member 5 which serves as a member for circulating these balls 4.
In a barrel portion of the nut 3 having a cylindrical shape, two internal-deflector windows 6 each having a substantially circular cross section are formed side by side so as to penetrate through the inner and outer peripheral surfaces of the barrel portion and to cut out part of the screw groove 3a. The internal-deflector member 5 serving as a member for circulating the balls 4 is fitted in each internal-deflector window 6.
On the inner side of the internal-deflector member 5, a connecting groove 5a for connecting adjacent rounds of the screw groove 3a to each other is formed. As a result, the connecting groove 5a and a portion of the screw groove 3a which constitute an approximately one round allow the ball 4 to orbit. That is, the rolling path formed of the screw grooves 2a, 3a facing each other is an orbiting path (circulation line). The large number of balls 4 interposed between the inner and outer screw grooves 2a, 3a inside the rolling path roll along the screw grooves 2a, 3a, are guided to the connecting groove 5a of the internal-deflector member 5, pass over a screw thread of the screw shaft 2, return to the adjacent screw groove 3a, and rolls again along the screw grooves 2a, 3a. 
The connecting groove 5a of the internal-deflector member 5 is curved in an S shape so as to smoothly connect the adjacent screw grooves 3a, 3a of the nut 3, and is smoothly connected to the screw groove 3a. In addition, the depth of the connecting groove 5a is set to a depth such that the ball 4 can pass over the screw thread of the screw groove 2a in the screw shaft 2 in the connecting groove 5a. 
In an assembly device of such a ball screw, as illustrated in FIG. 9, the internal-deflector member 5 is previously fixed to or integrally molded with the nut 3, a ball guide 10 is inserted in the nut 3, and a predetermined number of balls 4 are supplied from the ball guide 10 toward the screw groove 3a on the inner surface of the nut 3 (supplied into the connecting groove 5a of the internal-deflector member 5 in FIG. 9). The ball guide 10 is provided with a ball supply path 11 having a diameter substantially equal to the bottom diameter of the screw groove 2a of the screw shaft 2 and having as many circulation lines as the circulation lines of the screw groove 2a. An outlet 11a of the ball supply path 11 is set at the highest position of a raceway groove of the nut 3 forming a corresponding circulation line. The ball guide 10 aligns a predetermined number of balls 4 and consecutively supplies the balls 4 to the respective ball supply paths 11. A ball pusher 12 pushes the ball 4 that has not entered the circulation line raceway due to its own weight, and thus the nut 3 is filled with the balls 4.
In the case of the above conventional art, the ball 4 which has not entered the circulation line raceway due to its own weight is pushed by the ball pusher 12 to fill the ball 4 into the nut 3. However, when the ball 4 is filled by pushing the ball pusher 12, the ball 4 near the outlet 11a of the ball supply path 11 is pushed upward of the outlet 11a, and thus the ball 4 is stuck in a small space between an outlet upper edge 11b of the ball guide 10 and a screw inner circumference 3b (an inner circumference of the internal-deflector member 5 in FIG. 9) of the nut 3. Therefore, there is a possibility that assembly failure will occur.
Therefore, Patent Literature 1, which solves the above-described problem, discloses an assembly device of a ball screw including a stepped portion 13 which projects toward a nut inner circumferential side and is provided at an outlet upper edge portion of a ball supply path 11 of a ball guide 10, as illustrated in FIG. 10. By adopting such a configuration, it is possible to prevent a ball 4 from being stuck in the gap between the ball guide 10 and the nut inner circumferential side, and stable ball assembly is enabled.